Lightweight flexible imide foam and method of producing same

ABSTRACT

Described is a lightweight open-cell flexible foam having a density no greater than about 6 pounds per cubic foot, preferably less than 1.0 pound per cubic foot (pcf) comprising the reaction product of an organic polyisocyanate and a mixture of difunctional acids, acid anhydrides, and difunctional anhydrides in the presence of a tertiary amine triazine catalyst, which reactants during the manufacturing process generate carbon dioxide, wherein the ratio of imide to amide groups in the final product is greater than 1.

This is a division of U.S. patent application Ser. No. 07/379,496, filed 07/13/89, now allowed.

BACKGROUND OF THE INVENTION

This present application is concerned with the field of polyimide-polyamide and lightweight flexible foams. More specifically, the application is concerned with foams resulting from the reaction of organic acids/anhydrides and polyisocyanates.

U.S. Pat. No. 3,300,420 teaches the preparation of foam resins from aromatic anhydrides and isocyanates. A high polyimide content foam and a lightweight flexible foam is not fully described.

U.S. Pat. No. 3,620,987 describes the reaction of polyisocyanates, with acids and anhydrides in the presence of epoxide groups. It is desired to prepare poured in place foams using a particular combination of catalysts, in particular, materials which catalyst the reaction between the isocyanate group and an active hydrogen atom such as those utilized in synthesizing polyurethanes. Various epoxy resins are described as useful in the reaction with the isocyanate and the anhydrides.

U.S. Pat. No. 4,738,990 describes a fireresistant foam composed of predominantly amide and imide linkages prepared utilizing a cyclic phosphorous catalyst. Heavy shrinkage of the resulting foam produced is undesirable.

It has been found that utilizing the cyclic triazine catalyst of the present invention in conjunction with utilization of multiple acid/anhydride reactants, one can achieve a lightweight, open-cell flexible foam with low shrinkage caused by the generation of carbon dioxide during the manufacturing process.

SUMMARY OF THE INVENTION

Described is a lightweight open-cell flexible foam having a density no greater than about 6 pounds per cubic foot, preferably less than about 1.0 pound per cubic foot (pcf) comprising the reaction product of an organic polyisocyanate and a mixture of difunctional acids, aid anhydrides, and difunctional anhydrides in the presence of a tertiary amine triazine catalyst, which reactants during the manufacturing process generate carbon dioxide, wherein the ratio of imide to amide groups in the final product is greater than 1.

DETAILED DESCRIPTION OF THE INVENTION

In carrying out the process of the present invention, generally the reaction takes place in the absence of a solvent. The process is generally a two or three step process where the reactants are blended or reacted together at a lower temperature which, due to the exotherm, causes the temperature to rise. In one or two steps, approximately 10% to 80%, preferably 45% to 75% of the theoretical carbon dioxide is generated. The reaction mixture is removed from the reaction vessel, cooled, and ground to a fine powder. The powder material is further reacted to completion, the foaming step, by heating at a higher temperature from about 200° to 300° C., preferably to 225° to 260° C.

A particularly important aspect of the present invention is the triazine catalysts that are utilized. The catalyst is preferably N, N', N", tris(dimethylaminomethyl)-hexahydrotriazine; N, N', N" tris (dimethylaminoethyl)hexahydrotrizine; N, N', N" tris(diethylamino-ethyl)hexahydrotriazine; N, N', N" tris(diethylamino-ethyl)hexahydrotriazine; N, N', N" tris(diethylaminopropyl)hexhydrotriazine; N, N', N" tris (triphenyl) hexahydrotriazine; N, N', N"tris triethanol hexahydrotriazine; N, N', N" tris trimethyl hexahydrotriazine and the like. The amount of catalyst that is used ranges from about 0.05 to about 10% by weight of the reaction mixture, preferably 0.5 to about 5% by weight of the reaction mixture.

While the triazines are the preferred catalysts to be utilized in the present application, increased conversion efficiency to a desired imide amide such as cyclic phosphorous catalysts such as the five membered phospholenes or phospholanes and four membered phosphetanes. Such cyclic phosphorous catalysts are described in U.S. Pat. No. 4,738,990 which is hereby incorporated by reference.

Other catalysts that maybe employed on supplemental basis are the alkali metal oxide catalysts or the alkali metal alkoxides, phenoxides, naphthoxides, tertiary amines, such as tertiary aliphatic or aromatic amines or cyclic, fused tertiary amines e.g., triethylene diamine and the like. For a listing of such desireable catalysts, see U.S. Pat. No. 4,061,623 and 3,300,420 which are hereby incorporated by reference.

These supplemental catalysts maybe be utilized in the amount of 0.05 to about 5% by weight of the reaction mixture.

The acid/anhydrides that are utilized in the present invention are a mixture of at least two reactants. One reactant is a dianhydride while the other reactant is a dicarboxylic acid material or a material that contains both an anhydride group and a carboxylic acid group or one that can form a carboxylic acid group in situ. When the polyisocyanate reacts with the reactant containing an anhydride, the resulting reaction product is preferably an imide. In the case of a dianhydride, both anhydrides are converted to imides by virtue of the formation of carbon dioxide in situ. The second reactant which is a carboxylic group will react to form an amide with release of carbon dioxide.

Examples of polycarboxylic derivatives which can be employed as the free carboxylic acids or as intramolecular anhydrides formed from the same or different acids are: isophthalic acid, terephthalic acid, trimesic acid and phthalic acid. Examples of polycarboxylic derivatives which can be employed as the free carboxylic acids or intramolecular anhydrides thereof, are:

trimellitic acid and the anhydride thereof, pyromellitic acid and the dianhydride thereof, mellophanic acid and the anhydride thereof,

benezene-1,2,3,4-tetracarboxylic acid and the dianhydride thereof.

benezene-1,2,3-tricarboxylic acid and the anhydride thereof,

diphenyl-3,3',4,4'-tetracarboxylic acid and the dianhydride thereof,

diphenyl-2,2',3,3'-tetracarboxylic acid and the diahydride thereof,

naphthalene-2,3,5,7-tetracarboxylic acid and the dianhydride thereof,

naphthalene-1,2,4,5-tetracarboxylic acid and the dianhydride thereof,

napthalene-1,4,5,8-tetracarboxylic acid and the dianhydride thereof.

decahydronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydride thereof,

4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,4,6-tetracarboxylic acid and the dianhydride thereof,

2,6-dichloronaphthalene-1,4,5,8-tetracraboxylic acid and the dianhydride thereof,

2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic and the dianhydride thereof,

2,3,5,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid and the diahydride thereof,

phenanthrene-1,3,9,10-tetracarboxylic acid and the dianhydride thereof,

perylene-3,4,9,10-tetracarboxylic acid and the dianhydride thereof,

bis(2,3-dicarboxyphenyl) methane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl) methane and the dianhydride thereof,

1,1-bis(2,3-dicarboxyphenyl)ethane and the dianhydride thereof,

1,1,-bis(3,4-dicarboxyphenyl)ethane and the dianhydride thereof,

2,2-bis(2,3-dicarboxyphenyl) propane and the dianhydride thereof,

2,3-bis(3,4-dicarboxyphenyl) propane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl) sulfone and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)ether and the dianhydride thereof, ethylene tetracarboxylic acid and the dianhydride thereof, butane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

cyclopentane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

pyrrolidine-2,3,4,5-tetracarboxylic acid and the dianhydride thereof,

pyrazine-2,3,5,6-tetracarboxylic acid and the dianhydride thereof,

mellitic acid and the trianhydride thereof,

thiophen-2,3,4,5-tetracarboxylic acid and the dianhydride thereof, and

benzophenone-3,3',4,4'-tetracarboxylic acid and the dianhydride thereof.

Other anhydrides which may be employed in the practice of this invention are: the intermolecular anhydride of trimellitic acid 1,2-anhydride (see, for example, U.S. Pat. No. 3,155,687); the bisanhydrides disclosed in U.S. Pat. No. 3,277,117 e.g. 4,4'-ethylene glycol bis-anhydro trimellitate, and the diadducts of maleic acid or anhydride with styrene.

While any of the polycarboxylic acids and intramolecular or intermolecular anhydrides thereof defined and exemplified above can be employed in the preparation of the polymers of the invention, a preferred group of compounds for this purpose are intramolecular anhydrides which are derived from polycarboxylic acids having at least three carboxyl groups of which at least two carboxyl groups are attached directly to an aromatic nucleus in ortho position with respect to each other. A particularly preferred group of polycarboxylic acid intramolecular anhydrides are those selected from the class consisting of anhydrides having the following formulae: ##STR1## wherein at least one of R₁ and R₂ represents a group selected from the class consisting of carboxyl and the anhydride group

    --C)o)--o--C(o)--

wherein the carbon atoms of the latter are each attached to adjacent carbon atoms in an aromatic ring, and whereinR₁ and R₂ optionally and additionally may represent from zero to three substituents selected from the group consisting of halogen and lower-alkyl and A is a bridging group selected from the class consisting of lower-alkylene, carbonyl, sulfonyl and oxygen.

With respect to the polyisocyanate that may be employed in the present invention, listed below are useful polyisocyanates:

propylene-1,2-diisocyanate

butylene-1,2-diisocyanate

butylene-1,3-diisocyanate

hexamethylene diisocyanate

octomethylene diisocyanate

nonamethylene diisocyanate

decamethylene diisocyanate

2,11-diisocyanato-dodecane and the like

meta-phenylene diisocyanate

para-phenyleen diisocyanate

toluene-2,4-diisocyanate

toluene-2,6-diisocyanate

xylene-2,4-diisocyanate

xylene-2,6-diisocyanate

dialkyl benzene diisocyanates, such as methylpropylbenzene diisocyanate, methylethylbenzene diisocyanate and the like

2,2'-biphenylene diisocyanate

3,3'-biphenylene diisocyanate

4,4'-biphenylene diisocyanate

3,3'-dimethyl-4,4'-biphenylene diisocyanate, and the like

methylene-bis(4-phenyl isocyanate)

ethylene-bis-(4-phenyl isocyanate)

isopropylidene-bis-(4-phenyl isocyanate)

butylene-bis-(4-phenyl isocyanate)

hexafluoroisopropylidene-bis-(4-phenyl isocyanate), and the like

2,2'-oxydiphenyl diisocyanate

3,3'-oxydiphenyl diisocyanate

4,4'-oxydiphenyl diisocyanate, and the like

2,2'-ketodiphenyl diisocyanate

3,3'-ketodiphenyl diisocyanate

4,4'-ketodipheyl diisocyanate

2,2'-thiodiphenyl diisocyanate

3,3'-thiodiphenyl diisocyanate

4,4'-thiodiphenyl diisocyanate, and the like

2,2'-sulfonediphenyl diisocyanate

3,3'-sulfonediphenyl diisocyanate

4,4'-sulfonediphenyl diiscyanate, and the like

2,2'-methylene-bis-(cyclohexyl isocyanate)

3,3'-methylene-bis-(cyclohexyl isocyanate)

4,4'-methylene-bis-(cyclohexyl isocyanate)

4,4'-ethylene-bis-(cyclohexyl isocyanate)

4,4'-propylene-bis-(cyclohexyl isocyanate)

bis-(para-isocyanato-cyclohexyl) sulfide

bis-(para-isocyanato-cyclohexyl) sulfone

bis-(para-isocyanato-cyclohexyl) ether

bis-(para-isocyanato-cyclohexyl) diethyl silane

bis-(para-isocyanato-cyclohexyl) diphenyl silane

bis-(para-isocyanato-cyclohexyl) ethyl phosphine oxide

bis-(para-isocyanato-cyclohexyl) phenyl phosphine oxide

bis-(para-isocyanato-cylohexyl) N-phenyl amine

bis-(para-isocayanto-cyclohexyl) N-methyl amine

2,6-diisocyanato-pyridine

bis-(4-isocyanato-phenyl) diethyl silane

bis-(4-isocyanato-phenyl) diphenyl silane

dichloro-biphenylene diisocyanate, bis-(4-isocyanato-phenyl) ethyl phosphine oxide

bis-(4-isocyanato-phenyl) phenyl phosphine oxide

bis-(4-isocyanato-phenyl) N-phenyl amine

bis-(4-isocyanato-phenyl) N-methyl amine

3,3'dimethyl-4,4'-diisocyanato biphenyl

3,3'-dimethoxy-biphenylene diisocyanate

2,4-bis-(B-isocyanato-t-butyl) toluene

bis-(para-B-isocyanato-t-butyl-phenyl) ether

para-bis-(2-methyl-4-isocyanato-phenyl) benzene

3,3'-diisocyanato adamantane

3,3'-diisocyanato biadamantane

3,3'-diisocyanatoethyl-1,1'-biadamantane

1,2-bis(3-isocyanato-propoxy) ethane

2,2'-dimethyl propylene diisocyanate

3-methoxy-hexamethylene diisocyanate

2,5-dimethyl hexamethylene diisocyanate

5-methyl-nonamethylene diisocyanate

1,4-diisocyanato-cyclohexane

1,2-diisocyanato-octadecane

2,5-diisocyanato-1,3,4-oxadizole

OCN(CH₂)₃ O(CH₂)₂ O(CH₂)₃ NCO

OCN(CH₂)₃ S(CH₂)₃ NCO

OCN(CH₂)₃ N(CH₂)₃ NCO

polymethylene polyphenyl isocyanate biruets of the formula

    OCN-R.sup.6 -N[C(o)NH R.sup.6 NCO].sub.2

where R₆ is an alkylene group having 1-6 carbon atoms.

Mixtures of two or more of the above isocyanates can be used, such as mixtures of the 2,4- and 2,6-isomers of tolylene diisocyanate, mixtures of the 2,4'-and 4,4'-isomers of methylenebis(phenyl isocyanate) and the like. In addition to the 4,4'-methylenebis(phenyl isocyanate) or mixtures of the 2,4'-isomer and 4,4'-isomer thereof which are employed as the isocyanate component, there can also be used modified forms of these isocyanates. For example, there can be used 4,4'-methylenebis(phenyl isocyanate) or an admixture thereof with a minor amount of the 2,4'-isomer, which has been treated to convert a minor proportion, generally less than 15% by weight of the starting material. For example, the polyisocyanate and component can be methylenebis(phenyl isocyanate) which has been converted to a stable liquid at temperatures of about 15° C. and higher using, for example, the process described in U.S. Pat. Nos. 3,384,653, 3,394,164 and 3,394,165; polymethylene polyphenyl isocyanate mixtures wherein the methylenebis(phenyl isocyanate) content is from about 20 to about 85 percent by weight and the remainder of said mixture comprising polymethylene polyphenyl isocyanates of functionality higher than 2. In accordance with the preferred class of polyisocyanates the preferred illustrative polyisocyanates set forth above are those which fall into the aromatic diisocyanate category, most preferred are those falling int eh methylenebis(phenyl isocyanate) category.

The ratio of the number of equivalents for the reactants ranges from about 0.9 to about 1.1 NCO groups to a combination of acid anhydride groups, preferably a slight excess of isocyanate groups over acid plus anhydride.

The ratio of imide to amide groups in the final product is such that it is greater than a 1 to 1 ratio preferably approximately 1.1 to approximately 19 imide groups per amide group.

The foam that is produced in the process of the present invention is one that is a light weight open cell foam having a density of less than 6 pounds per cubit foot, preferably less than 1.0 pounds per cubic foot, and greater than 1 imide to amide groups.

The open cell foam may be obtained by the two stage reaction. In the first stage, the reaction is interrupted when between 10 and 80%, preferably 45 to 75% of the theoretical CO₂ has evolved. Then the reaction mass is removed from the first reaction stage to a second foaming stage. There the CO₂ generation may be completed.

The foam of the present invention is one that has increased fire retardancy compared to polyurethane foam. It may be used in such places as confined areas, e.g., aircraft cabins, below decks in ships and the like. The use of a high imide product gives the desirable thermal stability to the product.

The imide/amide foam of the present invention can be applied by adhering to desirable substrates or between walls. An adhesive may also be applied to a wall and the like.

The imide/amide foam of the present invention can be characterized as having the following properties: High thermal stability, low shrinkage, and low density.

Having described the invention, listed below are working examples wherein all temperatures are in degrees Centigrade and all parts are by weight, unless otherwise indicated.

EXAMPLE 1

Into a resin kettle containing a heating mantel, a stirrer for the reactants, an exit for carbon dioxide to be measured and a thermo-couple for recording the temperature, is added 0.42 equivalents for methylene diphenyl diisocyanate (MDI) and heated to 140° C under a nitrogen blanket with agitation. When the MDI reached 140° C., 0.38 equivalents of trimellitic anhydride (TMA) and 0.02 equivalents of benzophenonetetracarboxylicdianhydride (BTDA) were added. 0.70 grams of N, N', N" tris (dimethylaminopropyl) s-hexahydrotriazine (DMAPH) was added next. Reaction between the isocyanate and TMA and BTDA was evidenced by the release of CO₂ gas, which was measured as it passed through a wet test meter. After about 15% reaction as determined by the CO₂ volume released from the reactor, a silicone surfactant, L5430 (Trademark of Union Carbide), was added. After 20% of the total theoretical CO₂ was released, the mixer was turned off and the material was removed from the kettle and allowed to cool on an aluminum sheet.

After cooling, the 20% reacted material was placed in a 160° C. oven and allowed to heat approximately 15 minutes. This additional heating allowed the reaction to proceed to approximately 50-60% CO₂ generated of theoretical amount. After this material cooled, it was ground into a fine powder. Foam is prepared by placing the powder on the bottom of an aluminum tray and placing the tray in a convection oven set at approximately 230° C. The powder melts, continues to react liberating CO₂ which acts as the blowing agent expanding the polyamide/imide polymer to a foam. The foam is left in the oven about 40 minutes to complete foaming and to cure the foam sufficiently. The finished foam is lightweight and flexible. The theoretical imide to amide ratio of the fully reacted product is 52 imide groups/48 amide groups).

EXAMPLE 2

Utilizing a Henschel mixer, a blend was made consisting of mondur M, a flake MDI (551.3 grams), trimellitic anhydride (385.4 grams), BTDA (27.3 grams), 1, 3, 5 tris (dimethylaminopropyl) --s-- hexahydrotriazine (4.7 grams) and a surfactant L5430 (Trademark of Union Carbide). This blend was fed into the feed end of a laboratory single screw extruder. The extruder has 4 temperature zones; the first zone was set at 50° C., the second one at 170° C., the third set at 180° C., and the last zone at 190° C. The material exited the extruder at a temperature of 175° C. The output was measured at 3.6 pounds/hour. The mixture was allowed to cool and then prefoamed 10 minutes at 160° C. The material from the prefoam step was ground into a fine powder. Twenty grams of powder was placed uniformly on the bottom of a 4"×6" aluminum pan and foamed at 230° C. for 40 minutes. The resulting foam exhibited slight shrinkage, fine cells, no brittleness or friability. The theoretical imide to amide ratio of the fully reacted product is 52 imide groups/48 amide groups).

EXAMPLE 3

In a 3 liter reaction kettle fitted with a heating mantle 157.5 grams of fused methylene diphenyldiisocyanate (MDI) at 40° C. is added and heated under a nitrogen blanket with agitation. After the isocyanate reached 140° C., a blend of 110.1 g of trimellitic anhydride (TMA) and 7.8 g of benzophenonetetracarboxylic dianhydride (BTDA) was added. The temperature was Allowed to cool to 120° C. At 120° C., 0.90 g of N, N', N" tris (dimethylaminopropyl) --s-- hexahydrotriazine was added. After the triazine catalyst was added, the release of CO₂ increased. After 5.7 liters of CO₂ was released as determined by a wet test meter, 0.105 g of a silicone surfactant was added. When 7.8 liters of CO₂ was released from the reaction, the mixing was stopped and the mixed materials removed from the reactor kettle.

After cooling, the mixed material was further reacted by heating for 13 minutes in a convection oven at 160° C. After cooling, this material was ground into a fine powder. Foam was made by placing the powder uniformly on the bottom of an aluminum tray and placing into an oven set at 230° C. for 40 minutes. The resulting foam was lightweight, exhibited no shrinkage, brittleness, or friability. Table 1 lists physical property results of the foam. The theoretical imide to amide ratio of the fully reacted product is 52 imide groups/48 amide groups).

                  TABLE I                                                          ______________________________________                                                                Foam      Foam                                                                 Example 3 Example 4                                     Property    Test Method                                                                               Results   Results                                       ______________________________________                                         Density     --         0.55 pcf  0.60 pcf                                      Friability  ASTM       0.574%    3.97%                                                     C421                                                               Planar Tensile                                                                             ASTM       11.50 psi 4.59 psi                                                  D3574                                                              Compression ASTM       18.62%    23.76%                                        Set, 25%    D1667                                                              Compression ASTM       1.42 psi  1.63 psi                                      Deflection  D1056                                                              (25%)                                                                          ______________________________________                                    

EXAMPLE 4

In a Henschel mixer was added 551.3 g Mondue M (methylene biphenyl diisocyanate in flake form), 385.4 g trimellitic anhydride (TMA) and 27.3 g of benzophenotetracarboxylic dianhydride (BTDA) and mixed for approximately two minutes. 4.7 g of N, N', N" tris (dimethylaminopropyl) --s-- hexahydrotriazine and 1.75 g of a silicone surfactant was added and mixed for approximately 30 minutes.

The mixed material was allowed to react in a convection oven for 15 minutes at 160 C. The reacted material was ground into a fine powder and foamed at 230° C. for 40 minutes. The foam exhibited low friability, no brittleness, fine cells and very little shrinkage. The physical properties of this foam are listed in Table I. The theoretical imide to amide ratio of the fully reacted product is 52 imide groups/48 amide groups).

EXAMPLE 5

In a resin kettle was added 0.3633 equivalents of 143 L liquid isocyanate, 0.1733 equivalents of benzophenonetetracarboxylic diahydride and 0.1727 equivalents of trimellitic anhydride that was ground to a fine powder. After mixing for 3 minutes, 0.3 g of N, N', N" tris (dimethylaminopropyl) --s-- hexahydrotriazine was added and mixed for 3 minutes. Then, 0.14 g of a silicone surfactant was added wit an additional 3 minutes for mixing. The mixed material was removed from the resin kettle and performed for 7 minutes at 160° C. The prefoam was ground into a fine powder and foamed in an aluminum tray at 230° C. for 40 minutes. The resulting foam was fine celled, exhibited no shrinkage or brittleness or friability. Thermogravimetric Analysis (TGA) was performed on this foam as listed in Table II. The theoretical imide to amide ratio assuming full reaction, 3:1 imide: amide.

                  TABLE II                                                         ______________________________________                                         Thermogravimetric Analysis                                                     Sample Heated 5° C./min. in Air                                                       Temperature for Weight                                           Weight Loss, %                                                                               Loss to Occur, °C.                                        ______________________________________                                         10            503                                                              20            553                                                              30            569                                                              50            583                                                              ______________________________________                                    

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 

What is claimed is
 1. A lightweight open-cell flexible foam having a density no greater than 6 pounds per cubic foot (pcf) comprising the reaction product of an organic polyisocyanate having at least two functional groups per molecule and a mixture of a difunctional acid and a difuntional anhydride in the presence of a tertiary amine triazine catalyst, which reactants during the manufacturing process generate carbon dioxide in situ, wherein the reaction is interrupted when the theoretical amount of carbon dioxide generated is from about 10% to about 80% and wherein the reaction mass is transferred after the interruption step to a second stage wherein the carbon dioxide generation is completed and wherein the final product ratio of imide to amide groups is no greater than 19 to
 1. 2. The foam of claim 1 wherein the isocyanate is an aromatic diisocyanate.
 3. The foam of claim 2 wherein the acid is an aromatic acid anhydride and the second reactant is a dianhydride.
 4. The foam of claim 1 wherein the catalyst is a hexahydrotriazine catalyst used in the amount of 0.05 to 10% by weight of the reaction mixture.
 5. The foam of claim 1 wherein the density is no greater than about 1.0 pcf.
 6. The foam of claim 1 wherein the difunctional acid is an organic acid anhydride.
 7. A lightweight open-cell flexible foam having a density no greater than 6 pounds per cubic foot (pcf) consisting essentially of the reaction product of an organic polyisocyanate having at least two functional groups per molecule and a mixture of a difunctional acid and a difunctional anhydride in the presence of a tertiary amine triazine catalyst, which reactants during the manufacturing process generate carbon dioxide in situ, wherein the reaction is interrupted when the theoretical amount of carbon dioxide generated is from about 10% to about 80% and wherein the reaction mass is transferred after the interruption step to a second stage wherein the carbon dioxide generation is completed and wherein the final product ratio of imide to amide groups is no greater than 19 to
 1. 8. The foam of claim 7 wherein the isocyanate is an aromatic diisocyanate.
 9. The foam of claim 7 wherein the acid is an aromatic acid anhydride and the second reactant is a dianhydride.
 10. The foam of claim 7 wherein the catalyst is a hexahydrotriazine catalyst in the amount of 0.05 to 10% by weight of the reaction mixture. 